Investment over the last decade to adapt electronic components from rigid constructs to agile and flexible architectures is driven by the desire to create strain-tolerant and reprogrammable systems such as wearable and reconfigurable electronics. Few materials have shown as much promise to transform these diverse technology areas as room-temperature liquid metals. These highly-conductive fluids are far superior to solid electronic materials in their mechanical flexibility and ability to physically reconfigure circuitry. While mercury (Hg) is the traditionally-referenced room temperature liquid metal (TMP=−39° C.), its physiological and environmental toxicity restricts its use in nearly every application. Gallium liquid metal alloys (GaLMAs) are an alternative class of liquid metals, rising to prominence because of their superior properties, including non-toxic attributes, near-zero vapor pressures, and a unique ability to avoid leakage out of ruptured microchannels. Herein, the terms GaLMA, liquid metal, LM, gallium liquid, EGaIn, and Galinstan™ may be used interchangeably.
GaLMAs include numerous alloys of gallium, most notably eutectic gallium indium (EGaIn) and Galinstan™ (TMP=15° C. and −19° C., respectively), and form a thin, viscoelastic oxide layer on their surface in oxygen concentrations as little as 1 ppm. This oxide skin is common to all GaLMAs as gallium migrates to the surface of the liquid and rapidly oxidizes, forming oxides and sub-oxides of Ga with some hydroxyls present. The instantaneous oxide formation on GaLMAs provides unmatched capabilities to create self-supporting liquid structures in free space and prevents material loss in the case of microchannel damage, unlike any other class of fluids. The oxide skin also enables the liquid to remain within small diameter, high aspect ratio microchannels at ambient pressure and does not contribute noticeably to the interface resistance between the liquid metal and solid electrical contacts.
While the rapid oxidation of GaLMA surfaces can be extremely advantageous, the lack of control over this reactive gallium interface has been the primary cause of the fluid's poor implementation into flexible and reconfigurable electronics. The two critical obstacles are as follows:
GaLMAs tend to be very “sticky” because the surface oxide adheres to various substrates and microchannel walls, resulting in deposition of fluid onto random surface areas and in detrimental locations; and
Gallium aggressively alloys with every metal used in electronics, resulting in unwanted phase transitions, embrittlement of solid metal circuitry, and general lack of control of fluid location.
There is no known method of keeping the oxide and allowing it to repeatably flow through channels while maintaining a barrier between the liquid and solid metal states. The main approach reported to date to circumvent the negative “sticky” effects of the GaLMA fluids has been to use strong acids or bases to continuously etch away the oxide skin. While these methods have resulted in temporary removal of the “sticky oxide” problem, they have not yet led to practical integration of GaLMAs into useful applications for several reasons. In addition to introducing highly corrosive materials, e.g. hydrochloric acid, into an otherwise benign system, etching away the oxide forfeits all the beneficial attributes of GaLMAs detailed above. While under normal circumstances the diffusion of gallium into solid metals is slow, an acidic or basic environment substantially accelerates this diffusion process. To prevent the diffusion of gallium into other solid metal contacts, barrier layers are needed, which may detrimentally introduce additional contact resistive losses and processing steps. Better rheological control of the liquid metals in microfluidic channels and chemical stability at the liquid metal/electrode contacts is paramount to utilize these fluids to realize the game-changing capabilities in reconfigurable and flexible electronics.
Conductive traces in electronics have historically been composed of metals that are solid at room temperature, e.g. gold, silver, and copper. Lead and tin are common metals used in electronic solder. All commonly-used solder has negative characteristics including thermal expansion and contraction, which thermally limits their range of operation. There is also the concern of extremely thin crystalline growths, i.e. tin whiskers, which grow perpendicularly out from the surface of solder. Toxicity is also a concern with lead-based solder.
In electronics applications where flexibility is preferred (or essential) GaLMAs, e.g. gallium (Ga) and gallium alloys, are viable options although they too have negative characteristics. Gallium is a metal that is liquid near room temperature, making it very useful in the electronics industry, and it can maintain its intrinsic conductivity while being placed in flexible and stretchable components. Gallium and GaLMAs are also capable of self-healing, are reconfigurable, non-toxic, and stackable. These characteristics may be very useful, and both the self-healing and stackability features are due to the oxidation that occurs when gallium (Ga) and GaLMAs are exposed to oxygen (O). However, the oxide that forms can cause problems along with the benefits cited. Controlling the oxide that forms and mitigating the obstacles it presents is a challenge; current methods to overcome the obstacles have included the use of hydrochloric acid (HCl) or other acid/base chemistry which removes the oxide and its benefits, but which also adds an undesirable level of toxicity to the GaLMAs.